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A new classification of spin in systematic reviews and meta-analyses was developed and ranked according to the severity.

We aimed to (1) identify and classify spin (i.e., a description that overstates efficacy and/or understates harm) in systematic reviews and (2) rank spin in abstracts of systematic reviews according to their severity (i.e., the likelihood of distorting readers' interpretation of the results).First, we used a four-phase consensus process to develop a classification of different types of spin. Second, we ranked the types of spin in abstracts according to their severity using a Q-sort survey with members of the Cochrane Collaboration.We identified 39 types of spin, 28 from the main text and 21 from the abstract; 13 were specific to the systematic review design. Spin was classified into three categories: (1) misleading reporting, (2) misleading interpretation, and (3) inappropriate extrapolation. Spin ranked as the most severe by the 122 people who participated in the survey were (1) recommendations for clinical practice not supported by findings in the conclusion, (2) misleading title, and (3) selective reporting.This study allowed for identifying spin that is likely to distort interpretation. Our classification could help authors, editors, and reviewers avoid spin in reports of systematic reviews.